The present invention is directed to a method and apparatus for securing together two or more pieces of material in a manner that does not involve welding, adhesives, or the use of a separate fastener yet does involve the creation of a mechanical joint that has properties comparable to those produced by welding and separate fasteners.
Spot welds in aluminum sheet, such as Aluminum Alloys 6009 and 6010, have only approximately 60% of the static tensile strength and approximately 40% of the fatigue tensile strength of spot welds in steel sheet employed in automobile bodies. (Static strength is the highest constant load a joint will carry. Fatigue strength is the cyclic load a joint will carry over a given number of cycles.) This presents a serious deterrent to the use of aluminum body sheet in automobiles, as the method's alternative to spot welding of aluminum sheet presents substantial performance and cost problems in comparison to the spot welding of steel. Clinched joints, for example, which are alternative mechanical, non-separate fastener joints, have adequate static tensile strength but inadequate stiffness and fatigue tensile strength. Properly made adhesive joints have satisfactory properties, but the cost and the speed of production are problems since adhesives require cleaning of the sheet, spreading of the adhesive, jigging, clamping sheet portions together and then curing of the adhesive. The cost of the adhesive material is also a factor in the economics of using adhesives. Rivets give satisfactory performance but again the cost and speed of production are problems, as the rivets require jigging, drilling or punching of holes in the sheets, placing the rivet in the sheets and heading the rivet. The cost of the rivet itself is also an economic factor. By comparison, the spot welding of steel requires only jigging and the welding process which involves a single-stroke operation. The spot welding of steel does not require any preparation for joining nor the placement of an expendable joining element, such as a rivet, nor the step of fixing the joining element in place. This makes the spot welding of steel substantially faster, more convenient and lower in cost than the available methods for joining aluminum.